Hard fork
A hard fork is a non-backwards-compatible protocol change where new rules invalidate old, forcing all nodes to upgrade or stay on a split chain.
In a hard fork rules are relaxed or fundamentally changed: blocks/transactions valid under new rules are rejected by old nodes. If not everyone upgrades, two chains emerge (e.g. Bitcoin Cash split from Bitcoin in August 2017 via a hard fork). More common in Ethereum than Bitcoin. Bitcoin is conservative: prominent cases like BCH triggered intense debates over governance and the 'spirit' of Bitcoin.
Example
August 2017: Bitcoin Cash split by raising the blocksize limit from 1 MB to 8 MB — incompatible with Bitcoin. Miners picked sides; BTC holders received an equivalent amount of BCH on the new chain.
Frequently asked questions
What happens to my coins in a hard fork?
If you control your own keys: after split you have coins on both chains. At exchanges: depends on policy. Claim processes require care against replay attacks.
Can Bitcoin still see a hard fork?
Unlikely without extreme cause (quantum threat, necessary bug fix). The community treats hard forks as last resort. Contentious hard forks result in altcoins.
Related terms
Further reading
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